1. Field of the Invention
The present invention relates to a vehicle communication system in which multiple electronic control units mounted to a vehicle are connected with each other via a network to enable data transmission therebetween.
2. Description of Related Art
Some electronic control units (ECU) mounted to a vehicle include a volatile memory, which maintains data by being powered from a battery mounted to the vehicle. The ECU may determine, by mirror check or the like, whether an abnormality occurs in the volatile memory. If it is determined that the abnormality occurs in the volatile memory, the ECU initializes the data stored in the volatile memory, and records a memory initialization code indicating the initialization of the volatile memory. For this technique, see Patent Document 1 for example.
In the above way, reliability of the data stored in the volatile memory can be checked based on whether the memory initialization code is stored. More specifically, if the memory initialization code is stored, it can be determined that there is a question about reliability of the data stored in the volatile memory because the abnormality had occurred in the volatile memory storing the data. By contrast, if the memory initialization code is not stored, it can be determined that the stored data is reliable because the abnormality has not occurred in the volatile memory storing the data.
For example, when vehicle diagnosis information is stored in the volatile memory, it is possible to check the reliability of the stored vehicle diagnosis information based on whether or not the memory initialization code is stored. Thus, it is possible to suppress an occurrence of the following difficulty; an actually-faultless vehicle part is misidentified as an abnormal part and is replaced.    Patent Document 1: JP-H7-20007A
According to the technique disclosed in Patent Document 1, however, if an ECU is configured to start up upon switch-on of an ignition switch (IG switch) and determine whether an abnormality occurs in a volatile memory, the detachment and attachment of the battery during OFF of the ignition switch leads to, after the switch-on of the IG switch, a determination that the abnormality has occurred in the volatile memory, and the ECU initializes the volatile memory and records a memory initialization code. Thus, even if the memory initialization code is stored, it is impossible to determine whether the initialization of the volatile memory is caused by the abnormality of the volatile memory or caused by the detachment and attachment of the battery. Note that if the initialization is caused by the detachment and attachment of the battery, it should be determined that the data stored in the volatile memory is reliable because the initialization is not caused by a memory abnormality. Hereinafter, the initialization of the volatile memory caused by the detachment and attachment of the battery is also referred to as a battery detachment abnormality. An abnormality in a volatile memory is also referred to as a memory abnormality.
As can be seen from the above, when the memory initialization code is stored, it is necessary to consider a possibility of an occurrence of the memory abnormality even if this initialization of the volatile memory is actually caused by the detachment and attachment of the on-board battery. Therefore, the reliability of the date stored in the volatile memory cannot be determined.